Marine speedometers for measuring the velocity of a marine vessel traveling over water typically include a housing recessed in the hull or mounted on the transom of the marine vessel in which a paddlewheel is mounted. The paddles of the paddlewheel extend below the hull so that water flowing under the hull can turn the paddlewheel. The rotation of the paddlewheel is then converted into the speed of the vessel relative to the water. An example of such a marine speedometer is found in U.S. Pat. No. 3,706,224.
A drawback with this type of marine speedometer is that the protruding paddles of the paddlewheel can be damaged if struck by objects floating in the water or if pressed against the frame of a boat trailer when loaded onto the trailer.
One solution to this problem is to recess the paddlewheel flush with or above the level of the hull such that the paddlewheel does not extend beyond the surface of the hull. Such a marine speedometer is currently sold as part no. 278 00 276 by Bombardier Corporation (FIG. 1) which is fitted into the hull surface 62. In order to direct the water through the paddles of the recessed paddlewheel 64, the bombardier speedometer housing 66 includes a gradually recessed channel 68 about 1/2 inch deep at its deepest point and between about 3 1/2 inches and 4 inches long. The channel includes a curved transition portion 70 comprising two radii which joins a horizontal portion 72. The water passing under the speedometer follows the transition portion 70 of the channel 68 and then flows parallel to the hull 62 within the horizontal portion 72 of the channel 68 into the paddlewheel 64. The paddlewheel 64 projects below the upstream surface of the horizontal portion 72 of the channel 68. The flow regime of the water as it engages the paddlewheel is similar to that taught in U.S. Pat. No. 4,664,787. The Bombardier speedometer relies upon the pressure of the water under the hull caused by the weight of the boat on the water to direct the water along the channel. A drawback with this design is that the Bombardier speedometer requires two radii to recess the flow channel and, therefore, the speedometer is quite long making it impractical for transom mounting.